The present invention relates to calibrating laser trimming apparatus.
Laser trimmers are used to trim electrical components (e.g., resistors, capacitors or conductors) on semiconductor wafers used in integrated circuits. They typically employ a laser beam source, a laser beam positioning mechanism which directs the beam to a desired location on the semiconductor workpiece using mirrors that are precisely moved by galvanometers or x-y stage, and a lens, which focuses the beam.
Wells et al., "A Technique for Automatic Focusing and Alignment of Monolithic Circuits for Laser Trimming," Third Annual Microelectronics Measurement, Technology Seminar Proceedings (Mar. 17-18, 1981 San Jose, CA) discloses calibrating the positioning of a beam by moving a laser beam over a surface that is marked at known locations in a manner that affects reflectance of the beam and measuring the beam's reflectance at various galvanometer positions associated with nominal positions on the workpiece, using a photodetector that receives part of the reflectance back through the optical path.
As used herein, "nominal laser position" means the desired position of the laser on the workpiece; it is the position to which the laser beam positioning mechanism is instructed to place the laser. "Actual position" means the actual position of the laser on the workpiece.